Bimanual coordination between isometric contractions and rhythmic movements: an asymmetric coupling

Interactions between rhythmically moving limbs typically result in attracti on to a limited number of coordination modes, which are distinguished in te rms of their stability. In addition, the stability of coordination typicall y decreases with elevations in movement frequency. To gain more insight int o the neurophysiological mechanisms underlying these stability characterist ics, the effects of phasic voluntary muscle activation onto the movement pa ttern of the contralateral limb as well as onto the stability of interlimb coordination were examined. This was done in circumstances in which a minim al degree of movement-elicited afferent information was available to mediat e the coupling influences. The task involved rhythmic application of isomet ric torque by one hand, while the other hand was moving rhythmically with u nconstrained amplitude. The effects of two levels of applied torque, two co ordination patterns (inphase and antiphase), and two movement frequencies w ere determined, both at the behavioural level (movement kinematics and kine tics) and the neuromuscular level (EMG). The isometric applications of torq ue clearly influenced the muscle-activation profile and movement pattern of the other limb, affecting both temporal variability and amplitude. Surpris ingly, there were no differences between the two coordination patterns or b etween the tempo conditions. As such, the results did not conform to the Ha ken-Kelso-Bunz model for rhythmic movement coordination. These data suggest that the archetypal differences in stability of rhythmic bimanual coordina tion are contingent upon a correspondence between the limbs in terms of the ir respective tasks. This interpretation is elaborated in terms of the role of sensory feedback and the functional specificity of motor unit recruitme nt in rhythmic interlimb coordination.